The present invention relates broadly to gaskets for providing environmental sealing and/or electromagnetic interference (EMI) shielding, and more particularly to an electrically-conductive, tubular extrusion gasket profile which exhibits a controlled deflection response when compressibly deformed intermediate a pair of surfaces such as within an electronics enclosure.
The operation of electronic devices including televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. Such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed "electromagnetic interference" or "EMI" as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other "target" devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having a less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., "Gaskets That Block EMI," Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
As is shown in U.S. Pat. Nos. 5,603,514; 5,522,602; 5,512,709; 5,438,423; 5,524,908; 5,202,536; 5,142,101; 5,115,104; 5,105,056; 5,028,739; 5,008,485; 4,952,448; and 4,857,668, EMI shielding gaskets typically are constructed as a resilient core element having gap-filling capabilities which is either filled, sheathed, or coated with an electrically conductive element. The resilient core element, which may be foamed or unfoamed, solid or tubular, typically is formed of an elastomeric thermoplastic material such as polyethylene, polypropylene, polyvinyl chloride, or a polypropylene-EPDM blend, or a thermoplastic or thermosetting rubber such as a butadiene, styrene-butadiene, nitrile, chlorosulfonate, neoprene, urethane, silicone rubber, or fluorosilicone rubber.
Conductive materials for the filler, sheathing, or coating include metal or metalplated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, plated glass, or a conductive polymer material may be substituted.
Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, or die-cutting, with molding or die-cutting heretofore being preferred for particularly small or complex shielding configurations. In this regard, die-cutting involves the forming of the gasket from a cured sheet of an electrically-conductive elastomer which is cut or stamped using a die or the like into the desired configuration. Molding, in turn, involves the compression, transfer, or injection molding of an uncured or thermoplastic elastomer into the desired configuration.
Requirements for typical EMI shielding applications, and particularly those for tubular extrusion gasket profiles, generally specify a low impedance, low profile connection which is deflectable under normal closure force loads. Other requirements include low cost and a design which provides an EMI shielding effectiveness for both the proper operation of the device and compliance, in the United States, with commercial Federal Communication Commission (FCC) EMC regulations.
As revealed in U.S. Pat. Nos. 3,758,123, 4,968,854; 5,068,493; 5,107,070; and 5,578,790, Vanguard Products, Danbury, Conn., publication "Ultra-Vanshield," and in the Parker Chomerics, Woburn, Mass., publications: "EMI Shielding For Commercial Electronics" pp. 10, 14, 15, 17, and 25 (1996); EMI Shielding Engineering Handbook," pp. 42-47 (1989); "EMI Shielding for Military/Aerospace Electronics," pp. 39-51, 65 (1996); and Technical Bulletins 13 (1995), 20 (1997), 25 (1996), and 58 (1997), typical design for conventional gasket profiles include round or 0-shaped, square, rectangular, inverted D, P, or V-shaped, .OMEGA.-shaped, U-shaped, and various combination cross-sections. Heretofore, a D-shaped extrusion profile including a planar base portion and a rounded upper portion often was specified for certain sealing applications as the base portion afforded a generally flat interface surface useful for securing the gasket to an opposing mating surface of substrate with a pressure sensitive adhesive (PSA) or the like. With respect to tubular, i.e., hollow D-shaped extrusion profiles, however, it has been observed that, in a compressed or deformed orientation, these profiles exhibit a deflection response characterized by an upward lifting of a the planar base portion from the mating surface. This deflection, in reducing the contact area between the base portion and mating substrate, will be appreciated to correspondingly decrease both the overall EMI shielding and the environmental sealing effectiveness of the gasket. Moreover, this deflection further exerts a lifting force on any PSA interlayer tape which may ultimately produce an adhesive failure via a shear or peel mechanism.
In view of the foregoing, it will be appreciated that improvements in the design of tubular extrusion profiles for EMI shielding gaskets and the like would be well-received by the electronics and other industries. Especially desired would be an extrudable profile adapted for use even in low closure force application which maintains uniform contact with the base substrate for consistent EMI shielding and environmental sealing performance.